I played through the game entirely solo the first time through. It is not even remotely the same experience when played with an NPC helper, let alone with an actual player. I have been running with a couple of co-workers through the game recently, and the absolutely shit-show that is boss fights with -any- help is down right depressing. It is most definitely not the same experience, and you are really doing a disservice to yourself by using helpers on any boss fight, period.
The game is so much better when played solo. The boss fights are a joke when even -one- person is assisting you and isn't completely braindead.